There are situations in which it is desirable to remove a portion of casing or tubing from an oil or gas well. A typical situation may be to remove a length of casing to allow a permanent cement plug to be installed, prior to well abandonment. Current Oil and Gas UK Guidelines for the Abandonment of Wells (July 2015, Issue 5) dictate that a permanent barrier, typically a cement plug, must be formed between the reservoir and the seabed to act as one of a number of permanent barriers when a well is abandoned or plugged. This measure is intended to isolate the well and reduce the possibility of pressure migration in order to prevent hydrocarbons and other well fluids from underground reservoirs leaking past the barrier(s) and coming to surface and spilling into the sea.
In some situations, prior to installing the cement plug to abandon or plug the well, it is necessary to remove the production tubing, casing and other downhole tubulars, and the cement or other downhole fixings that secure the well to the bedrock.
Casing may also be removed to undertake a casing repair, or to expose the cement behind the casing to allow cement repair. In some cases, where cemented casing is used, for example, there may be a leak path in the cement behind the casing or between casing layers. Rectifying such a breach may also require the removal of a casing section and associated cement before forming new cement and repairing the casing.
Conventional removal of cemented casing uses, for example, milling tools or hydro-abrasive cutters which remove the casing and associated cement by gradually cutting or milling away small portions of metal and cement. These are slow processes and therefore make such an operation very expensive and time consuming.
Perforating charges have also historically been used to penetrate a casing wall, to allow fluid communication through the casing wall and to allow cementing behind. Perforations only produce small holes through the target, whereas large holes are often desirable.